The present invention relates to semiconductor processing and, more particularly, relates to an apparatus and method for measuring a real-time etching rate.
In the fabrication of integrated circuits, the semiconductor substrate or wafer is exposed to numerous process steps. One of the process steps includes etching the materials built up on the wafer to selectively remove certain portions to form the various features utilized in the fabrication of the integrated circuit. The portion removed is defined by a pattern generally formed using an organic photoresist mask. One type of etching process employs a dry chemistry that generally refers to the use of plasma having active species that react with the material to be etched, in order to volatilize and selectively remove the exposed portions. Another type of etching process (sometimes referred to as ashing) reacts with the photoresist to volatilize and strip the photoresist mask from the wafer.
A problem associated with plasma etching (ashing) processes is the difficulty in determining when the etch step has been completed. This difficulty occurs because plasma techniques are typically timed processes, based on predetermined etch rates. The predetermined etch rates are identified by performing a calibration step in which a relative etch rate is determined based on the amount of substrate removed during a known time interval. Since the exact conditions (i.e., pressure, gas flow, electric field, etc.) used during the calibration step may vary to some extent for the etch step during actual device fabrication, timed processes are inaccurate and only provide an estimate as to when the plasma etch process is completed. The time-based process does not provide a real-time etching rate.
As a consequence of the uncertainty in the time needed to etch a wafer, over-etching is used. This usually is defined as a fixed amount of time after which the etch is thought to be complete, in order to guarantee that the etching is complete over the entire wafer. Moreover, time-based processes typically require the use of dedicated equipment for thickness measurements, e.g., an ellipsometer. In order to determine a relative etching rate, before and after thickness measurement must be made thereby requiring operator intervention. More problematic is that the process introduces wafer-to-wafer variability since it is not a real time measurement of the etching rate.
In order to avoid the use of time-based processes for determining the endpoint of an etch step, diagnostic techniques have been developed which analyze the processes occurring in the reaction chamber. One such technique, called optical emission spectroscopy, monitors the intensity of the optical emission from both the plasma and the reactions on the wafer surface. The intensity of the optical emission is related to the concentration of molecular species generated. The completion of the etch process is determined when a change in the intensity of the optical emission is observed. A change in the intensity of the optical emission is observed when the concentration of particular molecular species being monitored is no longer present (or decreases dramatically) indicating that the layer responsible for reacting with the plasma to generate the particular monitored species has been removed. For example, an optical emission signal from hydroxyl species created during etching/ashing of a photoresist layer may be monitored to determine when the photoresist layer has been removed. Optical emission techniques require the reaction chamber to be equipped with an optical port for monitoring the optical emission.
One disadvantage of end-point systems is that the instantaneous ash rate is not known. Also, uncertainty in the end-point determination also requires over-etching.
Another disadvantage of present systems is that one cannot predict end-point. Some applications require the etch process to stop just short of end-point. This is particularly important for manufacturing of thin gate oxides. Unless one knows the instantaneous real-time etch rate, one cannot stop prior to completion with just an end-point system.
Optical interference is another known technique for etching rate measurement. A substrate or wafer with layers of thin material is illuminated with light of a known spectrum. Reflected light from the surface and material interfaces causes an interference pattern that is captured by an optical detector. The interference pattern behavior is determined by the differences in refractive index, thickness of the material being removed, wavelength and angles of incidence. As the thickness of the substrate changes, so does the interference pattern. This method requires the use of an external light source, usually a monochromatic light source such as a laser, dedicated equipment to collect, process and convert the optical interference pattern to a thickness measurement, and dedicated viewing ports in the reaction chamber for both incident and reflected/refracted light. However, the inclusion of such a system may not be a cost effective solution and as is most often the case, viewing ports cannot be arbitrarily located in the reaction chamber as this could impact on critical chamber geometries. For example, incident light and collection at an angle normal to the plane of the wafer requires the viewing port to be located in the same place as the source for the plasma/gas. Moreover, in process chambers using radiant heating to maintain the wafer at elevated process temperatures, the incident light for the optical interference diagnostic needs to be of considerable power so that reflected light is well above the strong background level emitted by the radiant heating source. However, use of such a laser can cause the substrate surface to locally overheat so that the local reaction rate deviates from the wafer average by a non-negligible amount. In this sense, the technique can no longer be considered non-perturbative.
Referring now to FIG. 1, there is shown a figure illustrating the general principles of optical interference for thin film coatings on reflective substrate materials. A semiconductor wafer 10 coated with a photoresist layer 12 having a thickness d and a refractive index n.
When an external light beam xcex is projected over the photoresist surface, light is both reflected and refracted from the surface. The reflected light beam (1) and refracted light beams (2, 3, 4, . . . ) travel different distances depending on the refractive index of the material comprising the surface and the thickness of the photoresist layer. Assuming the thickness d to be constant throughout the length of relevant refractions, the difference in the distance traveled by consecutive refracted beams (2, 3, 4 . . . ) is L. This relationship can be described mathematically as shown in equations (1) and (2). For a monochromatic light source xcex, the distance L corresponds to a phase shift xcex94xcfx86 between consecutive beams in accordance with well known optical principles. It should be noted that what really matters is the difference in xe2x80x9copticalxe2x80x9d path length and not just geometric path length, since the light ray travels more slowly in materials of higher index of refraction. The optical path length depends on both the geometric path length and the index of refraction along that path.
L=2d{square root over (n2xe2x88x92sin2)}xcex1xe2x80x83xe2x80x83(1)
xcex94xcfx86=2xcfx80(L/xcex)xe2x80x83xe2x80x83(2)
If the beams 1 and 2 are in phase with one another at the detector, the beams produce a constructive interference pattern, i.e., xcfx86=2kxcfx80 (k integer). Conversely, if the beams are out of phase, the beams will provide a destructive interference pattern, i.e., xcex94xcfx86=(2k+1)xcfx80. That is, a minimum for one of the beams coincides with a maximum for the other beam, or vice versa, thereby canceling or subtracting each other out.
When the beams are projected onto a target, e.g., a photo detector or optical fiber, the phase shift xcex94xcfx86 will cause the reflecting light intensity to vary from zero (destructive interference) to a maximum (constructive interference). The light intensity will vary between zero and the maximum depending on the phase of the different beams and create an interference pattern. The interference pattern will provide evidence of constructive and destructive interference. Mathematically, constructive interference is defined by Equation (3). In contrast, destructive interference, taking into consideration the light that is out of phase, can be mathematically defined by Equation (4).
xe2x80x83L=kxcex, kxcex5Nxe2x80x83xe2x80x83(3)
L=kxcex+xcex/2, kxcex5Nxe2x80x83xe2x80x83(4)
For incident angles close to normal, the contributions from 3rd order and higher reflections are extremely small compared to the 1st and 2nd order reflections and for practical purposes, can be ignored since these higher order reflections do not affect the interference pattern. For example, for a phase shift of xcex94xcfx86=(2k+1) xcfx80, consecutive reflected beams 1 and 2, 2 and 3, 3 and 4, etc. will be out of phase whereas beams 1 and 3, 2 and 4, etc. will be in phase. However, since the intensity of consecutive reflections decreases exponentially, the dominant interference is that between beams 1 and 2, which as noted will be out of phase and as such, will produce a destructive interference pattern.
The determination of the phase shift is more complicated than just the delay imposed by the alternate paths of the reflected ray and refracted rays. At each interface, there is a reflected and a refracted ray. Depending on the incident angles at each interface, the relative indices of refraction, and the polarization of the electric field, an additional phase shift is imposed which can vary from 0 to xcfx80 radians. The light source is composed of rays of light with all polarizations. Specifically, there is both an s-wave, with the electric field vector perpendicular to the plane of incidence, and a p-wave, with the electric field vector parallel to the plane of incidence. The plane of incidence is defined by the plane containing both the incident and reflected (refracted) wave propagation vectors. Note that the electric field for the s-wave is sometimes referred to as Exe2x8axa5and the electric field for the p-wave is sometimes referred to as the E∥. The phase changes at each interface can be different for the s-wave and the p-wave. Hence, each has to be accounted for separately. At the detector, the net resulting phase change will be the result of phase changes due to both the optical path length differences and the phase changes at each interface along the path length, as applicable.
FIGS. 2 (a through d) shows the changes in phase for each of the components of the electric field for the cases of light traveling from a low index into a high index of refraction material (FIGS. 2a and 2b) and light traveling from a high index into a low index of refraction material (FIGS. 2c and 2d). (Note that these figures happen to be for the case of an index of refraction of 1.5 for the higher index material.) One can see a large variation in behavior, depending on relative indices, electric field polarization, and angle of incidence to each interface. The polarization angles xcex8p and xcex8xe2x80x2p for the two cases are defined as arctan (nrel), where nrel=n2/n1 is the relative index or ratio of indices between the two materials, where n1 is the index for the material from which the light is incident. The critical angle xcex8c is defined as arcsin(nrel).
FIGS. 3 (a,b,c) show the amplitude coefficients for the reflected and transmitted waves for both light traveling from a low index into a high index of refraction material (FIGS. 3a and 3b) and light traveling from a high index into a low index of refraction material (FIG. 3c).
As the photoresist thickness d decreases, e.g., during a plasma mediated stripping (etching or ashing) process, the interference pattern cycles between constructive interference patterns and destructive interference patterns. The distance between two consecutive minima or two consecutive maxima (xcex94xcfx86=2xcfx80), corresponds to the change in thickness [xcex94d]one interference period, and can be mathematically described as shown in Equation (5).
[xcex94d]one interference period=xcex/(2{square root over (n2xe2x88x92sin2)}xcex1)xe2x80x83xe2x80x83(5)
Thus, for incident light normal to the plane of the wafer (xcex1=0), the change in thickness [xcex94d]one interference period is given by Equation (6).
[xcex94d]one interference period≈xcex/2n for xcex1≈0xe2x80x83xe2x80x83(6)
The sensitivity of thickness change to small angle variation is very low. For example, if the index of refraction for a photoresist is equal to 1.6 (n=1.6), a variation of 1xc2x0 in the angle (xcex1) causes less than 0.006% error. The sensitivity of thickness change [xcex94d]one interference period to small angle variation is mathematically shown in Equation (7). Variations in the viewing angle can occur, for example, as a result of wafer misalignment or tilting.                                                         Q              α                              Δ                ⁢                                  xe2x80x83                                ⁢                d                                      ≡                                                            ⅆ                                      (                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                      d                                        )                                                                    ⅆ                  α                                            ·                              1                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  d                                                              =                                                                      1                  2                                ·                                                      sin                    ⁢                                          xe2x80x83                                        ⁢                                          (                                              2                        ⁢                                                  xe2x80x83                                                ⁢                        α                                            )                                                                                                  n                      2                                        -                                                                  sin                        2                                            ⁢                      α                                                                                  ⁢                              xe2x80x83                            ⁢              for              ⁢                              xe2x80x83                            ⁢              α                        ≈            0                          ,                  
                ⁢                                            Q              α                              Δ                ⁢                                  xe2x80x83                                ⁢                d                                      ≡                          α                              n                2                                              ;                      xe2x80x83                    ⁢                                    for              ⁢                              xe2x80x83                            ⁢              α                        ≈                          π              2                                      ,                  xe2x80x83                ⁢                              Q            α                          Δ              ⁢                              xe2x80x83                            ⁢              d                                =                                                    π                /                2                            -              α                                                      n                2                            -              1                                                          (        7        )            
where xcex94d is assumed to stand for [xcex94d]one interference period.
The rate of thickness change is calculated by measuring the time between two consecutive minima or maxima. Thus, for incident light normal to the plane of the wafer (xcex1=0), if time xe2x80x9cTxe2x80x9d is the interference period, the stripping rate xe2x80x9cRxe2x80x9d can be determined by Equation (8).                     R        =                              [                                                            [                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    d                                    ]                                                  one                  ⁢                                      xe2x80x83                                    ⁢                  interference                  ⁢                                      xe2x80x83                                    ⁢                  period                                            T                        ]                    =                                                    λ                /                2                            ⁢              nT              ⁢                              xe2x80x83                            ⁢              for              ⁢                              xe2x80x83                            ⁢              α                        ≈            0                                                        (          8          )                ⁢                  xe2x80x83                    
As expected, the time resolution of the stripping rate R increases for short wavelengths since more interference periods will be observed per unit of time.
Optical interference normally requires the use of an external light source for providing the incident beams. Typically, the light sources include the use of laser diodes that emit monochromatic radiation at wavelengths greater than 600 nm. In radiantly heated reaction chambers, this particular wavelength range is not suitable since the radiant energy sources used to heat the wafers typically emit competing radiation. For example, tungsten lamps emit radiation from about 500 nm to about 2 xcexcm thereby competing with the radiation emitted by the incident beam. The background noise contributed by the radiant energy sources affects the interference pattern caused by the incident beam on the substrate surface. In order to minimize the effect of this background noise, the intensity of the incident beam must be of a magnitude sufficient to overcome the noise level in the system. However, increasing the intensity of the beam can alter the temperature at the point of focus for the incident beam. The increase in temperature will affect the striping rate and as such, the real time plasma mediated stripping rate for the bulk photoresist will not be accurate.
The problem is exacerbated in low temperature stripping processes in which the stripping rate is strongly temperature dependent. Increasing the temperature to overcome the background radiation can cause the local stripping rate to be significantly higher than the wafer""s average stripping rate across the wafer introducing an additional source of error in the observed rate.
It should be noted that the prior art processes fail to accurately depict the real-time etching rate. Rather, the prior art processes generally provide an average stripping rate. There accordingly remains a need in the art for an improved and robust process that is cost effective, accurately depicts a real time etching rate and minimizes the equipment dedicated to performing the rate measurements.
The present invention is directed to a process for determining a real-time stripping rate of a photoresist coating from a wafer. The process includes placing a wafer having a photoresist coating thereon into a plasma reaction chamber, wherein the reaction chamber includes a port. An optical detector is coupled to the port, wherein the optical detector includes receiving optics at a viewing angle nearly parallel to a plane of the wafer surface and fixedly focused at a focal point on a surface of the photoresist. A plasma comprising reactive species is generated and the photoresist is exposed to the reactive species. Interference signals are monitored and received by the optical detector, wherein the interference signals are produced from a direct light beam and light beams reflecting and refracting off the wafer. The direct, reflected and refracted light beams are generated within the chamber and are monitored at the same wavelength. An interference pattern is extracted from the interference signals and a real time etching rate R is calculated from the interference signals according to the relationship:
R=xcex/(2T{square root over (n2xe2x88x921)})
wherein xcex is the wavelength of the light beams generated in situ by the plasma or by a reaction between the photoresist and the reactive species or by an internal lamp used for heating the wafer; T is the time period interval between two consecutive minima in the interference pattern; and n is the refractive index of the photoresist measured at xcex.
An apparatus for measuring the real time plasma etching rate includes a plasma reaction chamber containing a port. An in situ light source within the reaction chamber illuminates a predetermined surface of the substrate, wherein the light source produces a direct light beam and a plurality of reflected and refracted light beams at the same wavelength. An optical detector is coupled to the port and includes receiving optics focused at the predetermined surface at an angle nearly parallel to the plane for receiving interference light signals produced by the direct, reflected and refracted light beams off the wafer. The apparatus includes computing means in communication with the receiving optics for computing the real time etching rate from the interference signals generated by the direct, reflected and refracted light beams. The computing means calculates the real time etching rate based on an interference pattern generated from the interference signals, the refractive index of the material to be removed and the selected wavelength of the direct, reflected and refracted light beams.